You are right I did forget to put the words 'for self defense' in. Coppers are trained to try and persaude people to admit it's for that: 'are you really saying if a gang came at you with a knives you wouldn't try and even the odds by using this?' no matter how unbelievable it is; swear blind you never would.
Course you could ask him 'well, would you?' if he's holding it and says 'yes' you can then execute a citizens arrest. he just admitted to a crime :-)

The rest of what you said; ok to carry for work; on the way from the shops etc I took as going without saying.
The Sock